The basic principles of refrigeration are derived from the behavior of fluid refrigerants when they change from a liquid to a gas or from a gas to a liquid and absorb or release latent heat to the environment. Such basic principles have long been utilized in ice making systems. Suitable fluid refrigerants include carbon dioxide and halogenated hydrocarbons.
An ice making system is typically made up of three principal components--a compressor, a condenser and an evaporator--which generally comprise a closed system. In normal operation, these components are usually run through a freeze or refrigeration cycle. Periodically, however, ice which accumulates or forms on the evaporator during the freeze cycle must be removed. Typically the accumulated ice is removed during a separate "harvest" or ice removal cycle.
During the freeze cycle, the compressor receives a vaporous refrigerant at low pressure and compresses it, thus increasing the temperature and pressure of the vaporous refrigerant. The compressor then supplies this high temperature, high pressure vaporous refrigerant to the condenser, where the refrigerant condenses, changing from a vapor to a liquid. In the process of condensing, the refrigerant releases heat to the condenser environment. From the condenser, the liquid refrigerant passes through the evaporator, where the liquid refrigerant changes state to a vapor. In the process of evaporating, the refrigerant absorbs latent heat from the surrounding environment. From the evaporator, the refrigerant flows to the compressor and the freeze cycle repeats itself. In a large ice making system, where a considerable amount of heat is released, the condenser is usually located far from the compressor, and typically outdoors.
In contrast to the freeze cycle, when an ice maker goes into "harvest," the refrigerant from the compressor is generally fed directly to the evaporator, rather than to the condenser. Because the heat from the vaporous refrigerant is used to melt and free the ice which has accumulated on the evaporator, ensuring an adequate transfer of heat from the compressor to the evaporator during the harvest cycle is a significant concern, particularly in those situations described below where an ice maker having a remote condenser is to operate over a range of ambient temperatures.
In many ice making systems which are designed to operate in low temperatures (e.g., below 50.degree. F. ambient), a head pressure control valve is provided to maintain a minimum head pressure to assure that compressor heat will be available for the ice harvest cycle. Such a head pressure control valve is generally designed to back-up liquid refrigerant in the condenser during cold temperatures. This back-up procedure, however, results in extra refrigerant charge being added to the system. As long as the system has enough receiver capacity, this extra refrigerant does not hurt the system during the freeze cycle as the outdoor temperature rises. However, when the ice maker shifts to the harvest cycle, this extra refrigerant can overload the compressor and damage the system.
During the harvest cycle, the vaporous refrigerant is supplied to the evaporators through a hot gas valve that typically has a fixed orifice which acts as a metering device. In self-contained systems with relatively small refrigerant charges, this works satisfactorily and provides acceptable harvest times without returning unacceptable amounts of liquid refrigerant to the compressor. However, in systems having large refrigerant charges (typically where the condenser is remote from the compressor), the discharge pressure during the harvest cycle tends to be much higher at elevated ambient (outdoor) temperatures than the discharge pressure in self-contained systems. This higher pressure causes more refrigerant to flow through the fixed orifice in the hot gas valve and into the cold evaporator where it condenses. If this condensed refrigerant subsequently reaches the compressor, the compressor can become slugged with liquid refrigerant and its efficiency can be materially impaired. The liquid refrigerent can also dissolve the lubricant in the compressor, resulting in harmful friction between its moving parts. Reducing the orifice size of the hot gas valve, however, would cause unacceptably long harvest cycles when the discharge pressures are lower, such as occur at lower outdoor temperatures.
In a preferred ice making system, a predetermined amount of refrigerant would be circulated between the compressor and the evaporator during the harvest cycle. The amount of refrigerant to be circulated would vary from system to system depending upon operating conditions, such as the ambient temperatures. In such an arrangement, the amount of refrigerant needed to ensure an efficient harvest cycle could be monitored by the compressor's suction pressure, and additional refrigerant could be added if needed. In this way, the ice maker would have the proper amount of refrigerant available during both the freeze and harvest cycles.